1. Technical Field
The present invention relates to a light emitting device and in particular to a light emitting device having a diffusing surface.
2. Description of the Related Art
Light emitting devices have been employed in a wide variety of applications, including optical displays, traffic lights, data storage apparatus, communication devices, illumination apparatus, and medical treatment equipment. How to improve the light emitting efficiency of light emitting devices is an important issue in this art.
Referring to FIG. 1, according to Snell's law, when a light is directed from one material with an index of refraction n1 towards another material with an index of refraction n2, the light will be refracted if its incident angle is smaller than a critical angle θc. Otherwise, the light will be totally reflected from the interface between the two materials. In other words, when a light beam generated from a light emitting diode (LED) travels across an interface from a material of a higher index of refraction to a material of a lower index of refraction, the angle between the incident light beam and the reflected light beam must be equal or less than 2θc for the light to be emitted out. It means that when the light generated from the LED travels from an epitaxial layer having a higher index of refraction to a medium having a lower index of refraction, such as a substrate, air and so on, a portion of the light will be refracted into the medium, and another portion of the light with an incident angle larger than the critical angle will be reflected to the epitaxial layer of the LED. Because the environment surrounding the epitaxial layer of the LED has a lower index of refraction, the reflected light can be reflected for several times inside the LED and finally a certain portion of said reflected light can be absorbed.
In U.S. Patent Publication No. 2002/0017652 entitled “Semiconductor Chip for Optoelectronics”, an epitaxial layer of a light-emitting device forming on a non-transparent substrate is etched to form a micro reflective structure having a multiplicity of semi-spheres, pyramids, or cones, then a metal reflective layer is deposited on the epitaxial layer. The top of the micro reflective structure is bonded to a conductive carrier (silicon wafer), and then the non-transparent substrate of the epitaxial layer is removed. All the light generated from the light emitting layer and incident to the micro reflective structure will be reflected back to the epitaxial layer and emitted out of the LED with a direction perpendicular to a light emitting surface. Therefore, the light will not be restricted by the critical angle any more.